1. Field of the Invention
The present invention relates to a charged-particle beam lithographic system for writing a desired pattern on a mask substrate or silicon wafer by an electron beam or ion beam.
2. Description of Related Art
An electron-beam lithographic system is used during a process in which a mask for creating semiconductor devices is fabricated. One example of this is a variable-area electron-beam lithographic system that writes a desired pattern by deflecting an electron beam while varying the cross-sectional area of the beam directed at a workpiece. In this system, the electron beam produced from the electron gun is directed at a first aperture having a rectangular opening. The beam transmitted through the first aperture is directed at a second aperture having a rectangular opening.
A shaping deflector is disposed between the first and second apertures. The electron beam can be shaped into an arbitrary cross-sectional area by deflecting the beam transmitted through the first aperture. An arbitrary pattern can be written on the workpiece by directing the shaped beam at the workpiece and deflecting the beam arbitrarily by the shaping deflector.
Where the shaped electron beam is deflected, if the deflection angle is increased, deflection distortion cannot be neglected. Therefore, the written region is divided into “fields” in which the deflection distortion of the beam can be neglected. The workpiece is moved field by field. Each part of a desired pattern is written on each field.
However, as lithographic patterns have become finer, slight deflection distortion and defocus within such fields have presented problems. Accordingly, as deflection is performed, defocus corresponding to the deflection angle needs to be corrected. It is also necessary to correct defocus due to Coulombic repulsive force varying according to variation of the size of the cross-sectional area of the illuminating beam, i.e., variation of the amount of current. Further, it is necessary to correct defocus corresponding to slight local height variations on the workpiece. A method of correcting defocus corresponding to the deflection angle is described, for example, in Japanese Patent No. 58005954. In this known method, the amount of correction is calculated based on distances x and y on the focal image plane due to deflection. The defocus is corrected by an auxiliary lens mounted close to the focusing lens.
Furthermore, an example of correction of defocus due to Coulombic repulsive force varying according to variation of the size of the cross-sectional area of the illuminating beam, i.e., variation of the amount of current, is described in Japanese Patent No. 08264420. This technique is variable-area charged-particle beam lithography in which correcting values corresponding to beam sizes are stored in a table for focus correction. During writing, a correcting value corresponding to a beam size is read from the table. A correcting signal corresponding to this correcting value is supplied to the focus-correcting lens. In this way, blur in the beam (i.e., defocus) due to variation of the beam size, i.e., variation of the current of the illuminating beam, is corrected.
However, as finer lithographic patterns are used, if defocus is corrected simply by using a focus-correcting lens, the following issues come to surface.
FIG. 2 is a principle beam diagram illustrating the issues. An object plane is located ahead of (on the left side of, as viewed in the figure) a lens and at a distance of a from the lens. It is assumed that an image at the object plane has a size of 1 and that an image at a rear image plane focused at a position at a distance of b from the lens has a size of m. If the distance from the lens to the image plane increases by Δb, the size of the image varies to m′. That is, the magnification of the image is affected by variation of the strength (excitation) of the lens.
In reality, this will present no problem in correcting defocus due to Coulomb repulsive force corresponding to the beam size, which is the object of Japanese Patent No. 08264420. However, where defocus is produced due to varying angle of deflection during deflection or due to height variations on the workpiece, if the defocus is corrected using a focus-correcting lens or the like, a problem arises.